1. Technical Field
The present invention relates to a technique of smoothing images.
2. Related Art
Among image processing techniques there is known a technique of smoothing an image, which is called ‘anti-aliasing’. As an example of this technique, a value of a target pixel is calculated on the basis of values of neighboring 16 pixels within a 4*4 matrix; to generate an output image based on pixel values already calculated. In a case that an image includes a character depicted in black and white, anti-aliasing calculates another color(s) between black and white to be used to depict an edge region of the character. Accordingly, stair-like lines that may appear at the edge of a character depicted with slanting lines, which are called ‘jaggies’, are changed to smooth straight or curved lines. Japanese Patent document JP-A-2004-287964 discloses a technique for performing anti-aliasing by use of hardware.
Generally, anti-aliased image data is temporarily stored in a Video Random Access Memory (VRAM) and then read from the VRAM to be displayed as an image on a display. When the display is capable of displaying an image by 16 gradations, each pixel has a value expressed by a 4-bit stream. Generally, a memory is configured to accept a 32-bit stream for a single writing process. If image data is written in a memory for each pixel (i.e. data is written by 4 bits), memory accesses have to occur frequently and thus writing data of a whole image in the memory takes considerable time to complete.
Alternatively, 32-bit data including pixel values for 8 pixels is written in a memory after performing of anti-aliasing for the 8 pixels, to reduce the number of memory accesses. In the latter case, the accessing time for writing data in the memory is shorter than in the former case. However, a significant amount of time is spent in performing the anti-aliasing 8 times before data can be written in the memory.